The Steyr ACR prototype, designed by Ulrich Zedrosser, was Steyr's entry for the United States Army's Advanced Combat Rifle program in the late 1980s/early 1990s. The Steyr was one of the top four ACR prototypes that were reviewed in Phase III, but none had managed to meet the army's requirement of 100% improvement over the M16A2 and therefore none were accepted.
Two prototype guns were manufactured. One of them, along with the other three finalist weapons, can be seen on exhibit at the National Infantry Museum in Fort Benning, Georgia, which is where they were also tested. Prototype #2 is kept at the Steyr Mannlicher arms factory in Austria.
The Steyr prototype is of a bullpup configuration, in which the internals, along with most of the barrel are placed within a smooth polymer body shell. It is also ambidextrous, due to having a bottomside ejection port ahead of the magazine well, eliminating ejection problems with left- or right-handed shooters. The weapon features a birdcage-type flash hider on the barrel. Drilled on the 1-o'-clock position of the flash hider are three rows of five recoil compensating holes each. The rifle also has an AR-15 style charging handle on the left-hand side, and fires from an open bolt.
The mechanism is a unique gas-operated system to cycle through ammunition; instead of driving rounds forward into the chamber and being held in place by a locking bolt, the entire chamber traveled vertically the width of the round. After firing, the expanding gases force the chamber vertically downward, where a new round was placed into it from the rear, ejecting the spent casing out the ejection port. Springs then raise the chamber back into position where it is locked into a fixed block. The firing pin is fixed above the chamber, entering through a small hole and striking the primer to fire. The chamber is normally held in the "down" position, and the trigger releases it to allow the springs to drive it upward and fire.
The safety is an ambidextrous button that can be pushed from either side of the weapon. On the left hand side of the button are red indication dots for the single-shot (one red dot) and three-round burst (two red dots) modes, and on the right is a white dot for safety.
A very similar mechanism is also seen in the ARES-Olin AIWS, which was an entrant in Phase I of the ACR program.
The weapon could be configured either for the standard 2x optic sight or traditional iron sights.
Ammunition is fed into the weapon from a 24- or 28-round, double column single-feed box magazine that is made of semi-translucent polymer.
The ACR's rounds consist of a 9.85 grain (.638 grams) carbon steel flechette using a four-part spindle sabot contained in a telescoped configuration with the propellant packed around the projectile. It also featured a side-initiating ring primer. Instead of brass, the bullet casings were made of a translucent polymer, reducing the weight by about half of that of the standard 5.56mm round. When fired, the flechette rode in the sabot until it had exited the barrel.
The ARES-Olin AIWS had also used telescoped ammunition.
The weapon had a tendency to malfunction and the accuracy and effectiveness of the flechettes left much to be desired, despite their flatter trajectory and lower recoil.
In addition, when the sabot leaves the barrel and disperses, the pieces would be a potential safety hazard to friendly troops or the shooter, with the pieces bouncing off of the ground when the weapon is fired in prone.
Along with that, due to inconsistencies in the strength of the plastic casings, the flechette tends to leave the cartridge at varying pressures. This, in turn, leads to varying muzzle velocities and changes in trajectory from shot to shot, negatively affecting accuracy. Steyr responded to this problem by experimenting with different materials and combinations of methods.
Another problem lays in the high production costs of the ammunition, meaning that very few countries are interested. In addition to this, is that even though spent casings eject from the bottomside of the ACR, the hot plastic can potentially burn the shooter in the wrists, so they would have to wear gloves to protect their wrists.
- Steyr ACR Article
- Wikipedia: Steyr ACR
- Modern Firearms - Steyr ACR - Advanced Combat Rifle (Austria)
- The Firearm Blog article "Steyr’s ACR: The ’80s-Era Teutonic Wonderwaffe You Haven’t Heard Of"
- The Firearm Blog: Steyr Mannlicher factory tour
- Forgotten Weapons article "Steyr ACR: A Polymer Flechette-Firing Bullpup From the 90s"